1. Field of the Invention
The present invention relates to a structure of a reflux fan for an excimer laser apparatus. More specifically, the present invention relates to a structure that supports and rotates a rotary shaft of a reflux fan for an excimer laser apparatus.
2. Description of the Background Art
A reflux fan for laser gas circulation in an excimer laser apparatus must have low vibration characteristic and durability. To meet such a demand, a magnetic bearing that realizes maintenance-free, non-contact support has been proposed for a bearing to be used in the reflux fan.
For example, U.S. Pat. No. 5,848,089 and Japanese Patent Laying-Open No. 11-303793 disclose examples of use of the magnetic bearing. In these references, a structure is disclosed in which a rotary shaft is supported in a non-contact manner by two radial magnetic bearings and an axial magnetic bearing consisting of an axial electromagnet, and a motor rotor fixed on the rotary shaft is driven to rotate by a motor stator on a stator side.
FIG. 41 is a cross sectional view showing a basic structure of a fan circulating excimer gas and peripheral portion thereof. In FIG. 41, a fan 203 is arranged in a chamber 201, and a laser gas is sealed inside the chamber 201. Fan 203 fixed on a rotary shaft 202 rotates in chamber 201. Magnetic bearings supporting rotary shaft 202 are arranged on opposite sides of chamber 201. On the left side, arranged are: a radial magnetic bearing 206 including a radial electromagnet 204 and a position sensor 205; an axial magnetic bearing 210 including axial electromagnets 207 and 208 and a position sensor 209; a motor 211 including a motor rotor 218 and a motor stator 217; and a protective bearing 212 as a touch down bearing that can support both the radial and axial directions to protect rotary shaft 202.
On the right side of FIG. 41, provided are: a radial magnetic bearing 215 including a radial electromagnet 213 and a position sensor 214; and a protective bearing 216 that can support only the radial direction.
Here, inner diameters of protective bearing 212, motor stator 217 and radial electromagnet 204 are adjusted to be approximately co-axial. A gap between the inner diameter of protective bearing 212 and rotary shaft 202 opposing thereto is set slightly smaller than a minimum dimension of the gap between the inner diameters of motor stator 217 and radial electromagnet 204 and the rotary shaft 202 opposing thereto, so as to prevent contact between rotary shaft 202 and radial electromagnet 204 or motor stator 214.
Axial magnetic bearing 210 and radial magnetic bearings 206 and 215 detect the position of rotary shaft 202 by means of position sensors 209, 205 and 214, respectively, provides signals by comparing operation between respective position sensor outputs and an instruction value, which signals are phase-compensated by a control circuit, not shown, and current-amplified by a power amplifier, so that a current is caused to flow in a coil of a corresponding electromagnet.
In the reflux fan for circulating laser gas in the excimer laser apparatus shown in FIG. 41, the pressure of reflux gas is as high as up to 5000 hPa. In order to rotate fan 202 under such a high output, it is necessary to increase output of motor 211 driving the rotary shaft 202. Because of high motor output, however, attraction between motor rotor 218 and motor stator 217 constituting motor 211 becomes undesirably strong. This not only increases disturbance on rotary shaft 202 containing motor rotor 218, but also affects control stability of radial magnetic bearing 206 supporting rotary shaft 202.
Support by the radial magnetic bearing must be controlled stably both in a state of non-rotation in which motor 211 does not have any influence and in a state of maximum rotation in which motor has significant influence. Further, rotation in every pressure range lower than the maximum value of 5000 hPa of the reflux gas pressure is necessary. Therefore, it has been difficult to ensure stability of control of the magnetic bearing.
FIG. 42 shows a structure near the radial magnetic bearing portion, and FIG. 43 is a block diagram of the magnetic bearing control system illustrating an influence of the motor on the radial magnetic bearing of FIG. 41.
Referring to FIG. 42, a desired distance between electromagnet 204 of the radial magnetic bearing and rotary shaft 202 is represented by X0, and displacement x from the distance X0 is measured. Based on the measurement, attraction force of electromagnetic 204 of the radial magnetic bearing is adjusted, so as to control floating position of rotary shaft 202. Here, motor 211 is positioned close to radial magnetic bearing 206, and control of radial magnetic bearing 206 is influenced by motor 211.
Referring to FIG. 43, P(s) represents an object of control of magnetic bearing itself not considering motor 211, G(s) represents transfer function of magnetic bearing control circuit, and x represents displacement from the position of prescribed floating distance X0 of the rotary shaft. After comparing operation between an output xxe2x80x2 of the position sensor detecting the displacement x of the rotary shaft with an instruction value r, an electromagnetic force Fa calculated by magnetic bearing control circuit G(s) consisting of a control circuit including proportional, integral and differential elements acts on rotary shaft 202, so that rotary shaft 202 is supported at a prescribed position. Here, Km represents a negative spring constant of motor 211.
Referring to FIG. 42, when rotary shaft 202 moves downward, attraction force Fm from motor stator 217 below increases, and spring constant attains seemingly negative. The value Km increases as the output of motor 211 increases, that is, as the attraction force between motor rotor 218 and motor stator 217 increases. In other words, control of the magnetic bearing must be designed in consideration of the value Km, while the value Km varies widely (when rotary drive is stopped, Km attains 0). Therefore, it is difficult to ensure stability of supporting the magnetic bearing in every state.
FIG. 44 represents a gain curve of Bode diagram of the object of control (in FIG. 43, transfer function from Fa to x), of the magnetic bearing when the motor is driven and not driven. In FIG. 44, solid line a represents gain curve when the motor is not rotating, while solid line b represents gain curve when the motor is being driven. It is understood that the gain curve lowers in a low frequency range, when the motor is driven. Because of this decrease in gain in the low frequency range and because of the characteristic that the gain curve is flat (gain frequency gradient is approximately 0) over a wide range in the low frequency range, controllability of the magnetic bearing degrades.
FIGS. 45A and 45B represent open loop transfer function when a magnetic bearing control circuit ensuring stability in both states (when motor is driven and not driven) is designed, based on the object of control of the magnetic bearing, in which FIG. 45A represents gain characteristic, and FIG. 45B represents phase characteristic.
Referring to FIG. 45A, solid line c represents the open loop transfer function when motor is not driven, while solid line d is when the motor is driven. When motor 211 is driven, gain margin shown in FIG. 45A decreases from A to Axe2x80x2, and it is understood that the margin in stability of control decreases significantly. As a countermeasure, it may be possible to set a cross over frequency to higher frequency side to suppress the influence of motor. In the excimer laser apparatus, however, discharge at a high voltage is utilized for laser oscillation, to excite laser gas. Therefore, in order to prevent influence of high frequency noise, in the magnetic bearing used for the excimer laser apparatus, it is necessary to lower as much as possible the control gain of the magnetic bearing.
Further, fan 203 used is long along the axial direction and as a result, rotation shaft 202 itself becomes longer, so that bending mode natural frequency of rotary shaft 202 lowers. In order to realize stable control thereof, it is also necessary to decrease the gain of the magnetic bearing control system. From these reasons, the method of setting the cross over frequency to a higher frequency side, which leads to increase gain in the high frequency range, is not desirable.
Thus, the reflux fan for laser gas circulation in the excimer laser apparatus has particular problems, that is, reflux gas pressure is as high as up to 5000 hPa, output of the motor 211 driving rotary shaft 202 to rotate fan 203 under such a high pressure attains high and, as a result, negative spring constant Km increases, that a source of high frequency noise is positioned in the vicinity, and that it is difficult to ensure control stability of the magnetic bearing, as rotary shaft 202 becomes long.
Further, as rotary shaft 202 is long, bending natural frequency of rotary shaft 202 becomes lower, degrading controllability of the magnetic bearing, and in addition, there arises another problem that rotary shaft 202 tends to bend.
Therefore, an object of the present invention is to provide a structure of a reflux fan for an excimer laser apparatus enabling stable magnetic bearing control, not increasing the gain of the magnetic bearing even when the motor is driven.
In summary, the present invention provides a structure of a reflux fan for an excimer laser apparatus in which laser gas in a chamber is circulated by rotation of a fan driven by a motor, wherein a rotary shaft which is rotated by a motor driving force and on which the fan is attached is supported in a non-contact manner by a control type magnetic bearing, the rotary shaft is supported by a protective bearing when the control type magnetic bearing is unable to support the rotary shaft, the control type magnetic bearing includes a radial magnetic bearing including three radial electromagnets placed at three positions along the axial direction and position detection sensors arranged near respective radial electromagnets, and an axial magnetic bearing including one axial electromagnet and at least one permanent magnet, the axial electromagnet is positioned opposing to one end surface of the rotary shaft and the permanent magnet is arranged opposing to the other end surface of the rotary shaft, the first radial magnetic bearing among the radial magnetic bearings is arranged near the axial electromagnet, the second radial magnetic bearing is positioned on one side to the rotary shaft than the permanent magnet arranged opposing to the end surface of the rotary shaft, and the third radial magnetic bearing is arranged between the motor and the fan.
Therefore, according to the present invention, two radial magnetic bearings are arranged on opposing sides of the rotary shaft, and the negative spring element of the motor is corrected by the motor side radial magnetic bearing. Thus, stable magnetic bearing control becomes possible while not increasing the magnetic bearing gain when the motor is driven.
More preferably, the protective bearing includes a first protective bearing supporting both axial and radial directions of the rotary shaft and arranged near the axial electromagnet and the motor, and a second protective bearing supporting only the radial direction of the rotary shaft and arranged near the other radial electromagnet.
More preferably, the structure further includes a third protective bearing arranged between the third radial magnetic bearing and the fan and capable of supporting only the radial direction.
More preferably, a soft magnetic material is provided at a portion where the rotary shaft opposes to the axial magnetic bearing, and except for the soft magnetic material, the rotary shaft has its diameter made equal to or smaller than the each inner diameter of the first radial magnetic bearing, the second radial magnetic bearing and the third radial magnetic bearing from the side of the axial electromagnet to the side of the permanent magnet.
Therefore, by removing from a housing the axial electromagnet and the first protective bearing or a housing for the protective bearing fixing the first protective bearing, and further removing the soft magnetic material of the rotary shaft opposing to the axial electromagnet, the stator and the fan from the rotary shaft, it is possible to remove the rotary shaft to the outside of the chamber and the housing, with the housing not removed from the chamber.
More preferably, the radial magnetic bearing has 8 magnetic poles in the circumferential direction, to generate electromagnetic force on the rotary shaft by adjacent to magnetic poles or, alternatively, includes four magnetic bearings in the circumferential direction and corresponding magnetic poles along the axial direction to be paired with the respective magnetic poles, and by a set of two magnetic poles adjacent in the axial direction, an electromagnetic force is caused to act on the rotary shaft, each radial magnetic bearing has two control axes and each control axis supports the weight of the rotary shaft itself.
According to another aspect, the present invention provides a structure of a reflux fan for an excimer laser apparatus in which laser gas in a chamber is circulated by rotation of a fan driven by a motor, wherein a rotary shaft rotated by the motor driving force is supported in a non-contact manner by a control type magnetic bearing, the rotary shaft is supported by a protective bearing when the control type magnetic bearing is unable to support the rotary shaft, the control type magnetic bearing includes a radial magnetic bearing including radial electromagnets arranged at two positions along the axial direction and position detection sensors arranged near respective radial electromagnets, and an axial magnetic bearing including one axial electromagnet and at least one permanent magnet, the axial electromagnet is arranged opposing to one end surface of the rotary shaft and the permanent magnet is arranged opposing to the other end surface of the rotary shaft, one radial electromagnet of the radial magnetic bearing is arranged close to the axial electromagnet, and the other radial electromagnet is arranged on the rotary shaft inner than the permanent magnet arranged opposing to the end surface of the rotary shaft.
Preferably, the protective bearing includes a first protective bearing arranged close to the axial electromagnet and the motor and supporting both the axial and radial directions of the rotary shaft, and a second protective bearing arranged near the other radial electromagnet and supporting only the radial direction of the rotary shaft.
Preferably, the structure further includes a third protective bearing arranged between the motor and the fan.
Preferably, at a portion of the rotary shaft opposing to the axial electromagnet, a soft magnetic material is provided, and except for the soft magnetic material, the rotary shaft has its diameter made equal to or smaller than the each inner diameter of the first radial magnetic bearing and second radial magnetic bearing from the side of the axial electromagnet to the side of the permanent magnet.
Thus, by removing the axial electromagnet and the first protective bearing or a housing for the protective bearing fixing the first protective bearing from a housing, and by removing the soft magnetic material of the rotary shaft opposing to the axial electromagnet, the stator and the fan from the rotary shaft, it becomes possible to remove the rotary shaft to the outside of the chamber and the housing, with the housing not removed from the chamber.
Further, according to another aspect, the present invention provides a structure for a reflux fan for an excimer laser apparatus in which laser gas in a chamber is circulated by rotation of a fan driven by a motor, wherein a rotary shaft rotated by a motor driving force and to which a fan is attached is supported in an non-contact manner by a control type magnetic bearing, the rotary shaft is supported by a protective bearing when the control type magnetic bearing is unable to support the rotary shaft, the control type magnetic bearing includes a radial magnetic bearing including radial electromagnets arranged at two positions along the axial direction and position detection sensors arranged around respective radial electromagnets, and an axial magnetic bearing including one axial electromagnet and at least one permanent magnet, the axial electromagnet is arranged opposing to one end surface of the rotary shaft and the permanent magnet is arranged opposing to the other end surface of the rotary shaft, the motor is arranged close to the axial electromagnet, the radial electromagnet of one radial magnetic bearing is positioned between the motor and the fan and the other radial electromagnet is positioned on inner side of the rotary shaft than the permanent magnet arranged opposing to the end surface of the rotary shaft.
Further, according to a still further aspect, the present invention provides a structure of a reflux fan for an excimer laser apparatus in which laser gas in a chamber is circulated by rotation of a fan driven by a motor, wherein a rotary shaft rotated by motor driving force and on which the fan is attached is supported in a non-contact manner by a control type magnetic bearing, and the rotary shaft includes a portion of an austenitic stainless steel, and a magnetic body portion fixed at a position on the surface of the austenitic stainless steel portion opposing to the electromagnet of the magnetic bearing.
More preferably, the rotary shaft is annealed at a temperature of at least 300xc2x0 C. with the magnetic body portion fixed.
More preferably, a material having Ni equivalent of (% Ni+30xc3x97% C+0.5xc3x97% Mn) of at least 16 and Cr equivalent given by (% Cr+% Mo+1.5xc3x97% Si+0.5xc3x97% Nb) of at least 18 is used for the rotary shaft.
According to a still further aspect, the present invention provides a structure of a reflux fan for an excimer laser apparatus in which laser gas is circulated in a chamber by rotation of a fan driven by a motor, wherein a rotary shaft rotated by a motor driving force is supported in a non-contact manner by a control type magnetic bearing, the rotary shaft is supported by a protective bearing when the control type magnetic bearing is unable to support the rotary shaft, and the control type magnetic bearing includes a radial electromagnet arranged in the axial direction of the rotary shaft and formed by sealing a coil with a metal that is corrosion resistant against the laser gas and a position detection sensor arranged around each radial electromagnet and formed by sealing a sensor unit with a metal that is corrosion resistant against the laser gas.
More preferably, the radial magnetic bearing includes a pair of magnetic bodies having a disk shape with a through hole formed at the center through which the rotary shaft is inserted, a plurality of coils arranged parallel to the axial direction between a pair of magnetic bodies, and a cylindrical member formed of a metal having corrosion resistance against the laser gas and sealing the periphery of the through hole of the magnetic body.
More preferably, the radial magnetic bearing includes a cylindrical magnetic body having protrusions on an inner portion, and a coil having its outer periphery sealed by a metal having corrosion resistance against the laser gas and inserted into the protrusion of the magnetic body.
Preferably, the radial magnetic bearing includes a ring-shaped nonmagnetic body, and a coil sealed by a metal tube having corrosion resistance against the laser gas and arranged at every prescribed angle, on the nonmagnetic body.
More preferably, the position sensor includes a disk shaped magnetic body having a through hole at the center through which the rotary shaft is inserted and a plurality of holes formed from the outer peripheral surface to the center, sensor unit inserted to the plurality of holes, and a cylindrical member formed of a metal having corrosion resistance against the laser gas and sealing the periphery of the through hole.
More preferably, the structure further includes a first housing provided on one side along the direction of the rotary shaft in the chamber, and a second housing provided on the other side, the radial electromagnet and the position detection sensor include a first radial electromagnet and a first position detection sensor provided in the first housing, and a second radial electromagnet and a second position detection sensor provided in the second housing.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.